Bluetooth: Mesh: Fix solicitation PDU tx dep on proxy

If `CONFIG_BT_MESH_GATT_SERVER` is disabled or the advertising set
doesn't support proxy adv, the solicitation PDU will not be sent.
However, solicitation PDU transmission doesn't depend on the proxy
feature of the device it sends. Therefore, solicatation PDU should be
sent regradless of `CONFIG_BT_MESH_GATT_SERVER` option and advertiser
tag.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>